Brushless motor for a power tool

ABSTRACT

A brushless motor includes a stator and a rotor rotatably received within the stator, the rotor including a rotor shaft, a rotor body mounted on the rotor shaft and including axial slots, permanent magnets disposed within the axial slots, and a rotor end cap provided at an end of the rotor body to axially retain the permanent magnets within the slots. The rotor end cap includes an outer planar portion that is in contact with the end of the rotor body, and ribs angularly extending from the outer planar portion towards a center portion forming openings therebetween to allow passage of air between adjacent openings in thermal contact with the end of the rotor body.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/907,664 filed June 22, which is a continuation of U.S. patentapplication Ser. No. 16/775,858 filed Jan. 29, 2020, now U.S. Pat. No.10,727,715, which is a continuation of U.S. patent application Ser. No.15/481,538 filed Apr. 7, 2017, now U.S. Pat. No. 10,587,163, whichclaims the benefit of U.S. Provisional Patent Application No.62/320,063, filed Apr. 8, 2016, all of which are incorporated herein byreference in their entireties.

FIELD OF THE DISCLOSURE

This disclosure relates to power tools. More particularly, the presentinvention relates to a power tool and a brushless motor for power tools.

BACKGROUND

Cordless power tools provide many advantages to traditional corded powertools. In particular, cordless tools provide unmatched convenience andportability. An operator can use a cordless power tool anywhere andanytime, regardless of the availability of a power supply. In addition,cordless power tools provide increased safety and reliability becausethere is no cumbersome cord to maneuver around while working on the job,and no risk of accidently cutting a cord in a hazardous work area.

However, conventional cordless power tools still have theirdisadvantages. Typically, cordless power tools provide far less power ascompared to their corded counterparts. Today, operators desire powertools that provide the same benefits of convenience and portability,while also providing similar performance as corded power tools.

Brushless DC (BLDC) motors have been used in recent years in variouscordless power tools. While BLDC motors provide many size and poweroutput advantages over universal and permanent magnet DC motors, it isalways desired to manufacture more compact motors while providing thesame or higher power output.

BLDC motors are available as canned motors, where all the motorcomponents are securely assembled inside a cylindrical motor can ormotor housing. The motor housing includes piloting features for therotor end bearings to retain the rotor assembly securely within thestator. The motor housing is encapsulated inside a power tool via twopower tool housing halves.

Alternatively, BLDC motors may be without a motor housing or can, wherethe stator/rotor assemblies are mounted directly inside the power tool.Such motors are typically provided with two end bearing support mountsprovided at the two ends of the stator assembly. The bearing supportmounts are axially fastened together on the stator via screws located onthe outer surface of the stator. The bearing support mounts constraintthe axial movement of the rotor within the stator. The bearing supportmounts also typically include radial retention features, for exampleradial constraints that partially wrap around the outer surface of thestator, to constraint the radial movement of the rotor within thestator. Radial retention features have to be manufactured with greatprecision to ensure that an air gap is provided between the rotor andthe inner surface of the stator.

U.S. patent application Ser. No. 13/919,352 (Publication No.2013/0270934), which is incorporated herein by reference in itsentirety, describes an example of a BLDC motor without a motor housing.As shown in FIGS. 2A and 2B of this disclosure, the two bearing supportmembers (i.e., a ring gear mount and a hall board mount assembly, alsocommonly referred to as motor caps) and the stator all include fastenerreceptacles that allow the three components to be securely fastenedtogether. Additionally, the two bearing support members include pilotingand retention semi-cylindrical walls that partially cover the outerdiameter (OC) of the stator lamination stack. These features radiallyretain the two bearing support members, and consequently the rotorassembly, with respect to the stator.

While these fastening and piloting features are important in precise andsecure assembly of the rotor with respect to the stator, they add to theoverall outer diameter of the motor. In particular, the piloting andretention walls add to the diameter of the stator lamination stack.Also, the screws receptacles add to the outer diameter of the stator andthe two bearing support members. In BLDC motors, particularly inhandheld portable power tools where space is limited, it would begreatly desirable to construct these piloting and retention features ina way that does not affect the length and diameter of the motor.

SUMMARY

According to an embodiment of the invention, a brushless direct-current(DC) motor is provided comprising a stator assembly and a rotorassembly. In an embodiment, the stator assembly includes agenerally-cylindrical stator body having a center bore, teeth extendingfrom the stator body towards the center bore and defining slots inbetween, and windings wound around the teeth. In an embodiment, therotor assembly is rotatably received within the center bore of thestator assembly, and includes a rotor shaft and a generally-cylindricalrotor body mounted on the rotor shaft. In an embodiment, the motorfurther includes at least one rotor bearing mounted on the rotor shaft,and at least one bearing support member supporting the rotor bearing. Inan embodiment, the bearing support member includes a radial body forminga bearing pocket at central portion thereon for receiving the rotorbearing therein, and axial post inserts received within the slots of thestator assembly between adjacent sets of windings and engaging an innersurface of the stator body to support the rotor bearing with respect tothe stator assembly along a center axis of the center bore of the statorassembly so as to maintain a circumferential gap between the rotor bodyand the stator teeth within the center bore of the stator assembly.

In an embodiment, a rear bearing and a front bearing are disposed at twosides of the rotor body. In an embodiment, a first bearing supportmember is provided supporting the rear bearing and a second bearingsupport member is provided supporting the front bearing.

In an embodiment, the radial body of the bearing support member includesa mating surface that mates with an end portion of the stator assemblyto form a substantially uniform cylindrical body between the statorassembly and the bearing support member.

In an embodiment, the bearing support member supports a circuit board onwhich positional sensors are mounted. In an embodiment, the positionalsensors are arranged to sense a magnetic position of the rotor assembly.

In an embodiment, the bearing support member includes a series ofopenings formed around the bearing pocket to allow passage of airthrough the bearing support member. In an embodiment, a fan is mountedon the rotor shaft facing the bearing support member. The fan generatesairflow that passes through the stator assembly and the openings of thebearing support member.

In an embodiment, the axial post inserts generally radially extend froma peripheral portion that is arranged to engage the inner surface of thestator body, to an end portion that is arranged at an open end of acorresponding slot and engages the edges of two corresponding statorteeth. In an embodiment, the axial posts include a generally rectangularcross-sectional profile.

In an embodiment, the stator assembly includes an end insulator arrangedat an end surface of the stator body to insulate the stator teeth fromthe windings. In an embodiment, the radial body of the bearing supportmember includes a mating surface that mates with a corresponding matingsurface of the end insulator to form a substantially uniform cylindricalbody between the stator assembly and the bearing support member.

In an embodiment, the mating surfaces of the end insulator and thebearing support member include corresponding indentations and detentsarranged to mate to properly align the bearing support member withrespect to the stator assembly.

In an embodiment, the rotor bearing is positioned along approximately asame radial plane as at least one of the end insulator or ends of theplurality of stator windings.

In an embodiment, the bearing support member is configured to be fullyslidingly received within the stator assembly.

According to an embodiment of the invention, a power tool is providedthat includes a housing, and a motor as described above disposed withinthe housing.

In an embodiment, an inner surface of the power tool housing includes aplurality of piloting and retaining features configured to axiallysupport the stator assembly and the bearing support member with respectto one another.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of this disclosure in any way.

FIG. 1 depicts an exemplary power tool having a brushless DC motor,according to an embodiment of the invention;

FIGS. 2A and 2B respectively depict front and rear respective views ofan exemplary brushless DC motor, according to an embodiment;

FIGS. 3A and 3B respectively depict front and rear exploded views of themotor, including a stator assembly, a rotor assembly, a first bearingsupport member, and a second bearing support member, according to anembodiment;

FIG. 4 depicts a perspective view of the first bearing support member ofthe motor, according to an embodiment;

FIG. 5 depicts a perspective view of the second bearing support memberof the motor, according to an embodiment;

FIG. 6 depicts a perspective view of a first sub-assembly including therotor assembly and the first bearing support member, according to anembodiment;

FIG. 7 depicts a perspective view of a second sub-assembly including thestator assembly and the second bearing support member, according to anembodiment;

FIG. 8 depicts a perspective radially-cut-off view of the motor,including the first bearing support member assembled on one side of thestator assembly and the rotor assembly, according to an embodiment;

FIG. 9 depicts a perspective axially-cut-off view of the motor,according to an embodiment;

FIG. 10 depicts a perspective partially-exploded view of the power tooland the motor, according to an embodiment;

FIG. 11 depicts a cut-off top perspective view of the power tool and themotor, according to an embodiment;

FIG. 12 depicts a perspective view of a brushless DC motor, according toan alternative embodiment of the invention;

FIG. 13 depicts a partially exploded perspective view of the motor,including the stator assembly and the rotor assembly, according to anembodiment;

FIG. 14 depicts an exploded view of a sub-assembly including the rotorassembly, and first and second bearing support members, according to anembodiment;

FIGS. 15A and 15B depict front and rear perspective views of the secondbearing support member, according to an embodiment;

FIG. 16 depicts a perspective view of a brushless DC motor, according toyet another alternative embodiment of the invention;

FIG. 17 depicts a partially exploded perspective view of the motor,including the stator assembly and the rotor assembly, according to anembodiment;

FIG. 18 depicts a perspective view of a sub-assembly including the rotorassembly, and first and second bearing support members, according to anembodiment;

FIG. 19 depicts an exploded view of the rotor assembly and the first andsecond bearing support members, according to an embodiment; and

FIGS. 20A and 20B depict front and back perspective views of a rotor endcap, according to an embodiment.

DETAILED DESCRIPTION

The following description illustrates the claimed invention by way ofexample and not by way of limitation. The description clearly enablesone skilled in the art to make and use the disclosure, describes severalembodiments, adaptations, variations, alternatives, and uses of thedisclosure, including what is presently believed to be the best mode ofcarrying out the claimed invention. Additionally, it is to be understoodthat the disclosure is not limited in its application to the details ofconstruction and the arrangements of components set forth in thefollowing description or illustrated in the drawings. The disclosure iscapable of other embodiments and of being practiced or being carried outin various ways. Also, it is to be understood that the phraseology andterminology used herein is for the purpose of description and should notbe regarded as limiting.

With reference to the FIG. 1 , a power tool 100 constructed inaccordance with the teachings of the present disclosure is illustratedin a longitudinal cross-section view. The power tool 100 in theparticular example provided may be an impact wrench, but it will beappreciated that the teachings of this disclosure is merely exemplaryand the power tool of this invention could be a drill, impact driver,hammer, grinder, circular saw, reciprocating saw, or any similarportable power tool constructed in accordance with the teachings of thisdisclosure. Moreover, the output of the power tool driven (at leastpartly) by a transmission constructed in accordance with the teachingsof this disclosure need not be in a rotary direction.

The power tool shown in FIG. 1 may include a tool housing 102 thathouses a motor assembly 200 and a control module 106, an input unit(e.g., a variable speed trigger) 110, and a transmission assembly 114having a gear case (not shown). The motor assembly 200 may be coupledthrough the gear case to an output spindle (not shown), which isrotatably coupled to a square wrench 107. The tool housing 102additionally includes handle 112 that, in an embodiment, houses thecontrol module 106.

According to an embodiment, motor 200 is disposed in housing 102 abovethe handle 112. Motor 200 may be powered by an appropriate power source(electricity, pneumatic power, hydraulic power). In embodiments of theinvention, the motor is a brushless DC electric motor and is powered bya battery pack (not shown) through a battery receptacle 111, though itmust be understood that power tool 100 may alternatively include a powercord to receive AC power from, for example, a generator or the AC grid,and may include the appropriate circuitry (e.g., a full-wave orhalf-wave bridge rectifier) to provide positive current to the motor200.

In an embodiment, input unit 110 may be a variable speed trigger switch,although other input means such as a touch-sensor, a capacitive-sensor,a speed dial, etc. may also be utilized. In an embodiment, variablespeed trigger switch may integrate the ON/OFF, Forward/Reverse, andvariable-speed functionalities into a single unit coupled and partiallymounted within control unit 106 and provide respective inputs of thesefunctions to the control unit 106. Control unit 106, which receivesvariable-speed, on/off, and/or forward/reverse signal from the inputunit 110, supplies the drive signals to the motor 200. In the exemplaryembodiment of the invention, the control unit 106 is provided in thehandle 112. It must be understood that while input unit 100 is avariable-speed unit, embodiments of the invention disclosed hereinsimilarly apply to fixed-speed power tools (i.e., tools without a speeddial or speed trigger, having constant speed at no load).

In an embodiment, brushless motor 200 depicted in FIG. 1 is commutatedelectronically by control unit 106. Control unit 106 may include, forexample, a programmable micro-controller, micro-process, digital signalprocessor, or other programmable module configured to control supply ofDC power to the motor 200 and accordingly commutate of the motor 200.Alternatively, control unit 106 may include an application-specificintegrated circuit (ASIC) configured to execute commutation of the motor200. Using the variable-speed input, forward/reverse input, on/offinput, etc., from the input unit 110, control unit 106 controls theamount of power supplied to the motor 200. In an exemplary embodiment,control unit 106 controls the pulse width modulation (PWM) duty cycle ofthe DC power supplied to the motor 200. For example, control unit 106may include (or be coupled to) a series of power switches (e.g., FETs orIGBTs) disposed in a three-phase inverter circuit between the powersource and the motor 200. Control unit 106 may control a switchingoperation of the switches to regulate a supply of power to the motor200, via motor wires 109.

Commutation details of the brushless motor 200 or the control unit 106are beyond the scope of this disclosure, and can be found in co-pendingInternational Patent Publication No. WO 3081/1596212 by the sameassignee as this application, which is incorporated herein by referencein its entirety. An example of an integrated switch and control moduleembodying an input unit 110 and a control unit 106 described herein maybe found in application Ser. No. 14/6210,617 filed Mar. 30, 3085 by thesame assignee as this application, also incorporated herein by referencein its entirety.

A first embodiment of the invention is described herein with referenceto FIGS. 2A-11 .

FIGS. 2A and 2B depict two perspective views of a brushless DC (BLDC)motor 200, according to an embodiment of the invention. FIGS. 3A and 3Bdepicts perspective exploded views of the same motor 200, according toan embodiment. As shown in these figures, the exemplary motor 200 is athree-phase BLDC motor having a rotor assembly 210 rotatably receivedwithin a stator assembly 230. Various aspects of motor 200 are describedherein. It must be noted that while motor 200 is illustratively shown inFIG. 1 as a part of an impact driver, motor 200 may be alternativelyused in any other device or power tool. Further, while motor 200 is athree-phase motor having six windings, any other number of phases orwinding configurations may be alternatively utilized.

In an embodiment, rotor assembly 210 includes a rotor shaft 212, a rotorlamination stack 214 mounted on and rotatably attached to the rotorshaft 212, and rear and front bearings 220, 222 arranged to secure therotor shaft 212, as discussed below. In an embodiment, rear and frontbearings 220 and 222 provides radial and/or axial support for the rotorshaft 212 to securely position the rotor assembly 210 within the statorassembly 230.

In various implementations, the rotor lamination stack 214 can include aseries of flat laminations attached together via, for example, aninterlock mechanical, an adhesive, an overmold, etc., that house or holdtwo or more permanent magnets (PMs) therein. The permanent magnets maybe surface mounted on the outer surface of the lamination stack 214 orembedded therein. The permanent magnets may be, for example, a set offour PMs that magnetically engage with the stator assembly 210 duringoperation. Adjacent PMs have opposite polarities such that the four PMshave, for example, an N-S-N-S polar arrangement. The rotor shaft 210 issecurely fixed inside the rotor lamination stack 214.

In an embodiment, rotor assembly 210 also includes a sense magnet 216attached to an end of the lamination stack 214. Sense magnet 216includes a similar magnetic arrangement as the rotor permanent magnetsand may be made of, for example, four magnet segments arranged in anN-S-N-S polar arrangement that align with the rotor permanent magnets.The sense magnet 216 is disposed in close proximity to and is sensed viaa series of positional sensors (such as Hall sensors), which providepositioning signals related to the rotational position of the rotorassembly 210 to control module 106. In an embodiment, sense magnet 216additionally axially limits the movement of the magnets on one end ofthe rotor lamination stack 214. In an embodiment, on the other end ofthe rotor lamination stack 214, a rotor end cap 226 is disposed, whichalso axially limits the movement of the magnets, described later indetail in this disclosure.

In an embodiment, a fan 218 is mounted on and rotatably attached to adistal end of the rotor shaft 212. Fan 218 rotates with the rotor shaft212 to cool the motor 200, particularly the stator assembly 230. In anembodiment, a pinion 205 may be disposed on the other distal end of theshaft 212 for driving engagement with the transmission assembly 114.

According to an embodiment, stator assembly 230 includes a generallycylindrical lamination stack 232 having a center bore configured toreceive the rotor assembly 210. Stator lamination stack 232 includes aplurality of stator teeth extending inwardly from the cylindrical bodyof the lamination stack 232 towards the center bore. The stator teethdefine a plurality of slots therebetween. A plurality of stator windings234 are wound around the stator teeth. The stator windings 234 may becoupled and configured in a variety of configurations, e.g.,series-delta, series-wye, parallel-delta, or parallel-wye. The statorwindings 234 are electrically coupled to motor terminals 238. Motorterminals 238 are in turn coupled to the power switch inverter circuitprovided in (or driven by) control module 106. Control module 106energizes the coil windings 234 via the power switch inverter circuitusing a desired commutation scheme. In an embodiment, three motorterminals 238 are provided to electrically power the three phases of themotor 200.

In an embodiment, front and end insulators 236 and 237 may be providedon the end surfaces of the stator lamination stack 232 to insulate thelamination stack 232 from the stator windings 234. The end insulators236 and 237 may be shaped to be received at the two ends of the statorlamination stack 232. In an embodiment, each insulator 236 and 237includes a radial plane that mates with the end surfaces of the statorlamination stack 232. The radial plane includes teeth and slotscorresponding to the stator teeth and stator slots. The radial planefurther includes axial walls that penetrate inside the stator slots. Theend insulators 236 and 237 thus cover and insulates the ends of thestator teeth from the stator windings 234.

According to an embodiment, motor 200 is additionally provided with twobearing support members 250 and 270 formed as motor caps disposed at andsecured to the two ends of the stator assembly 230, as described herein.In an embodiment, both bearing support members 250 and 270 are made ofinsulating (e.g., plastic) material molded in the structural formdescribed herein.

As shown in the perspective view of FIG. 4 and with continued referenceto FIGS. 2A-3B, first bearing support member 250 (also referred to asrear bearing mount or the hall board mount) provides structural supportfor rear rotor bearings 220, as well as printed circuit board 260 formounting the positional sensors (herein referred to as hall board 260).In an embodiment, first bearing support member 250 includes asubstantially planar radial body 254 forming a first bearing pocket 252,which in this example is a through-hole in the center of the planarradial body 254. The rear rotor bearing 220 is positioned and securedinside the first bearing pocket 252 via, for example, heat-staking,insert-molding, clamping via a small fastener, or other known method. Inan embodiment, the rotor shaft 212 is press-fitted inside the rearbearing 220 during the assembly process after the rear bearing 220 issecured inside the first bearing pocket 252.

In an embodiment, the fan 218 is disposed on the rotor shaft 212adjacent the first bearing support member 250 opposite the rotorassembly 210. First bearing support member 250 includes several openings256 formed between respective teeth 258 around the first bearing pocket252 that allow passage of airflow generated by fan 218 through the motor200.

In an embodiment, the rear surface of the first bearing support member250 facing the fan 218 acts as a baffle for the fan 218 and directs theair coming from the motor 200 into the fan 218 radially away from therotor shaft 212, thus significantly preventing the airflow from flowingback into the stator assembly 230 from the fan side. In an embodiment,the rear surface of the first bearing support member 250 includessemi-circular walls 251 around the circumference of the fan 218 tocontrol the flow of the outgoing air, for example, through an exhaustvent in the power tool housing 102.

In an embodiment, hall board 260 is mounted on an inner surface of thefirst bearing support member 250 facing the rotor assembly 210. In anembodiment, hall board 260 includes three hall sensors (or otherpositional sensors) 262 arranged around the first bearing pocket 252,and a hall terminal 264 accessible at or outside the periphery of thehall board mount 250. In an embodiment, first bearing support member 250includes retaining features 266 (e.g., snap features) for securelyretaining the hall board 260.

As shown in the perspective view of FIG. 5 and with continued referenceto FIGS. 2A-3B, second bearing support member (also referred to as frontbearing mount) 270 provides structural support for front rotor bearings222, in an embodiment. In an embodiment, second bearing support member270 includes a substantially planar radial body 274 defining a secondbearing pocket 272. Second bearing pocket 272 in this example includes apocket in which front bearing 222 of the rotor assembly 210 is securelyreceived, and a through-hole 273 having a smaller diameter than thepocket through which the rotor shaft 212 extends out. In an embodiment,the front bearing 222 is first mounted (e.g., via press-fitting) on therotor shaft 212 during the rotor assembly process. The front bearing 222is then received inside the second bearing pocket 272 during the fullmotor assembly process.

In an embodiment, second bearing support member 272 includes severalslots 276 formed between respective teeth 278 that allow passage ofairflow generated by fan 218 into the stator assembly 230 from a frontalside of the motor 200.

It is noted that the terms “rear” and “front” as they relate to thebearings or other motor components are relative to the positioning ofthe components with respect to motor output connected to thetransmission assembly 114.

As previously discussed, in conventional BLDC motors without a motorhousing, the two bearing support structures that support the rotorbearings with respect to the stator include piloting and retentionsemi-cylindrical walls that partially cover the outer surface of thestator lamination stack. These features provide radial alignment for therotor with respect to the stator. The bearing support members alsoinclude peripheral through-holes and fastening receptacles for fasteningthe bearing support members, either to the stator, or one another, overthe outer diameter of the stator lamination. The fasteners provide axialalignment for the rotor with respect to the stator. Presence of thesefeatures in the bearing support members results in increased overallouter diameter of the motor assembly.

In order to reduce the overall diameter of the motor, according to anembodiment of the invention, bearing support member piloting andretention features are provided on the inner-diameter (ID) of the statorlamination stack as described herein.

In an embodiment, as shown in FIGS. 4 and 5 , first and second bearingsupport members 250 and 270 are provided with axial post inserts 280 and290 shaped to be received within the slots of the stator laminationstack 232 between respective adjacent stator windings 234. In anembodiment, first bearing support member 250 includes six axial postinserts 280 projecting from the planar radial body 254 around the firstbearing pocket 252. Similarly, second bearing support member 270includes six axial post inserts 290 projecting from the planar radialbody 274 around the second bearing pocket 272. In an embodiment, axialpost inserts 290 include a generally rectangular cross-sectional profileextending from a peripheral portion 292, which is arranged to engage aninner surface of a corresponding lamination stack slot, to an endportion 294, which may be slightly thicker than the peripheral portion292 and is arranged to be disposed at an open end of the laminationstack slot, between an in engagement with two adjacent stator toothedges. In an embodiment, axial post inserts 280 have a similarconstruction with a generally rectangular cross-sectional profileextending from a peripheral portion 282, which is arranged to engage aninner surface of a corresponding lamination stack slot, to an endportion 284, which may be slightly thicker that the peripheral portion282 and is arranged to be disposed at an open end of the laminationstack slot, between and in engagement with two adjacent stator toothedges. In an embodiment, two of the post inserts 281 adjacent the hallboard 260 may include a shortened end portion to accommodate the hallboard 260.

It is noted that while in the illustrated embodiment, the axial postsinserts 280 and 290 are generally-rectangular shaped engaging an innersurface and two tooth edges of the stator lamination slots, it isenvisioned that axial members with various other shapes and engagingother surfaces of the stator lamination slots are within the scope ofthis disclosure.

During the assembly process, as shown in the perspective view of FIG. 6, rotor assembly 210 is first assembled with the first bearing supportmember 250 to provide a first sub-assembly. In this step the rotor shaft212 is press-fitted into the rear rotor bearing 220. The axial postinserts 280 are in this manner located at a circumferential distantaround the rotor lamination stack 214.

Additionally, as shown in the perspective view of FIG. 7 , statorassembly 230 is assembled with the second bearing support member 270 toprovide a second sub-assembly. In this step, the axial post inserts 290are tightly pressed into the stator slots, against the frictional forceof the inner surface of the lamination stack 232, between the statorwindings 234, until mating surfaces of the front end insulator 236 andthe second bearing support member 270 come into contact.

Once these steps are completed, the first sub-assembly is assembled intothe second sub-assembly to form the motor 200. In this step, the axialpost inserts 280 of the first bearing support member 250 are tightlypressed into the stator slots, against the frictional force of the innersurface of the lamination stack 232, between the stator windings 234 andopposite the axial post inserts 290 of the second bearing support member270, until mating surfaces of the rear end insulator 237 and the firstbearing support member 250 come into contact. The front rotor bearing222 is also form-fittingly received inside the bearing pocket 272 of thesecond bearing support member 270, with the rotor shaft 212 and thepinion 205 extending through the through-hole 273.

Referring to FIGS. 3A, 3B, 5 and 7 , in an embodiment, each endinsulator 236 and 237 includes various peripheral indentations 231 anddetents 239. In an embodiment, a mating surface of the first and secondbearing support member 270 includes corresponding detents 250/297 andindentations 293/298. During assembly, these indentations and detentsare lined up for proper alignment and piloting of the stator assembly230 and the two bearing support members 250 and 270, and engage oneanother to form a substantially uniform cylindrical body.

In an embodiment, the first bearing support member 250 includes one ormore flexible posts 285, made of resiliently elastic material such asrubber, axially extending its mating surface 291. When the rotorassembly 210 is received inside the stator assembly 230, flexible posts285 of the first bearing support member 250 come in contact with andpress against the rear end insulator 237. Flexible posts 285 account forand absorb any tolerances associated with the stator assembly 230, therotor assembly 210, or the first bearing support member 250, relative toone another.

In an embodiment, the second bearing support member 270 also includesone or more flexible posts 275, made of resiliently elastic materialsuch as rubber, axially extending from its end surface opposite thestator assembly 230. As discussed below in detail, when the motor 200 isassembled inside the power tool housing 102, flexible posts 275 absorbany tolerances associated with the stator assembly 210, the secondbearing support member 270, or the power tool housing 102, relative toone another.

FIG. 8 depicts a perspective radially-cut-off view of the motor 200,including the first bearing support member 250 assembled on one side ofthe stator lamination stack 232 and rotor lamination stack 214. As shownherein, in an embodiment, stator teeth 240 of the stator laminationstack 230 extending inwardly from the cylindrical portion 242 of thelamination stack 232 towards the center bore and define slots 244therebetween. Stator windings 234 are wound around the stator teeth 240within adjacent slots 244. Peripheral portions 282 of axial post inserts280 nest against an inner surface of the cylindrical portion 242 of thelamination stack 232 within the slots 244. End portions 284 of axialpost inserts 280 engage adjacent tooth edges 246 of adjacent statorteeth 240 for added support. In this manner, the peripheral portions 282are firmly held against the inner surface of the cylindrical portion 242of the lamination stack 232, constraining the lateral and/or radialmovement of the first bearing support member 250 with respect to thestator assembly 230.

FIG. 9 depicts a perspective axially-cut-off view of the motor 200,according to an embodiment. This cut-off view is provided along a planeintersecting the center of the motor shaft 212 and opposing axial postinserts 280 and 290. As shown herein, the axial post inserts 280 and 290project partially within the stator lamination stack 232 slots. In anembodiment, the axial post inserts 280 and 290 may each project toapproximately a quarter-way to a half-way point within the statorlamination stack 232. Axial post inserts 280 and 290 are firmlysupported by the stator lamination stack 232, and thus restrain thelateral and/or radial movement of the bearing support members 250 and270 with respect to the stator lamination stack 232.

As shown in FIGS. 8 and 9 , this arrangement ensures that the rotorlamination stack 214 is radially secured inside the stator laminationstack 232 and a substantially-uniform air gap is maintained between theouter circumference of the rotor lamination stack 214 and the statorassembly teeth 240 with a high degree of precision.

In an embodiment, the rear and front bearings 220, 222 axially restrainand secure the bearing support members 250 and 270 on the two sides ofthe rotor assembly 210 and the stator assembly 230. In addition, asdescribed herein, the power tool housing 102 and the motor 200 may beprovided with retaining and piloting features to help locate and securethe motor 200 within the power tool 100. These retaining and pilotingfeatures provide additional axial support to the motor 200 components.

FIG. 10 depicts a partially-exploded perspective view of the power tool100 with motor 200 shown at a distance from housing half 102. FIG. 11depicts a cut-off top perspective view of the power tool 100. Theretaining and piloting features of the tool housing 102, andcorresponding tabs, projections, or recesses of the motor 200 componentsthat engage the retaining and piloting features of the tool housing 102,are described herein with reference to these figures, and with continuedreference to FIGS. 2-7 .

In an embodiment, first bearing support member 250 includes two opposinggenerally-rectangular peripheral tabs 304 (one of which is shown in FIG.10 . Tool housing 102 includes corresponding channels or recesses 320(only one of which is shown in FIG. 10 ) arranged to receive theperipheral tabs 304 on both sides of the motor 200 when fully assembled.

In addition, in an embodiment, end insulator 237 of the stator assembly230 facing the first bearing support member 250 includes two pairs ofopposing U-shaped walls 308 (one pair being shown in FIG. 10 ) in closeproximity or in contact with the peripheral tab 304. U-shaped walls 308form recess portions 310 therein. Tool housing 102 includescorresponding posts 324 on both sides of the motor 200 that, whenassembled, are received inside recessed portions 310 and engage theU-shaped tabs 308. In an embodiment, posts 324 and U-shaped tabs 320 maybe provided with elastic pads 330 and 332 to account for smalltolerances associated with the motor 200 components.

In addition, in an embodiment, the tool housing 102 is further providedwith inner ribs walls 326 that, when fully assembled, engage two edges271 of the second bearing support member 270 on both sides of the motor200.

These piloting and retaining features 320, 324, and 326 of the powertool housing 102 not only help proper placement and alignment of themotor within the power tool 100, they provide axial constraints againstthe first bearing support member 250, the stator assembly 230, and thesecond bearing support member 270. These axial restraints reinforce theaxial restraints provided by the front and rear bearings 222 and 220.

The above-described embodiments of the invention reduce the overalloutside diameter (OD) of the motor. Alternatively, given the same spacelaminations inside the power tool housing, a motor according toembodiments of the invention can be provided with a larger OD statorlamination stack, providing more torque and power.

Table A below provides a comparison between two exemplary conventionalBLDC motors (without a motor can or motor housing) having piloting andretention features and fasteners on the outer surface of the statorassembly (1st with a 48 mm stator lamination stack OD, and the 2^(nd)with a 51 mm stator lamination stack OD), and an improved BLDC motorwith a 51 mm stator lamination stack OD having inner diameter (ID)piloting and retention features and no fasteners according toembodiments of this disclosure.

TABLE A 1st Conv. 2nd Conv. Improved BLDC BLDC BLDC Stator Lamination OD  48 mm   51 mm    51 mm First (Fan-Side) Motor Cap 54.4 mm   58 mm 54.4mm (Bearing Support Member) Second Motor Cap 53.5 mm 57.4 mm   51 mm(Bearing Support Member) Screws (threads) 50.2 mm 54.9 mm n/a

As shown in this table, in the first exemplary conventional BLDC motorwith 48 mm stack lamination stack OD, the diameters of the two motorcaps (i.e., bearing support members), as measured between opposingfastening receptacles, are 54.4 mm and 53.5 mm respectively. Thus, themotor caps increase the diameter of the motor by approximately 10-15%.The diameter of the stator, as measured between opposing screws on theoutside surface of the lamination stack, is also increased byapproximately 2 mm to 50.2 mm.

In the second exemplary conventional BLDC motor with 51 mm stacklamination OD, the diameters of the two motor caps (i.e., bearingsupport members), as measured between opposing fastening receptacles,are 58 mm and 57.4 mm respectively. Thus, the motor caps once againincrease the overall diameter of the motor by approximately 10-15%. Thediameter of the stator, as measured between opposing screws on theoutside surface of the lamination stack, is also increased byapproximately 4 mm to 54.9 mm.

It was found by inventors of this application that removing the screwsand the associated fastening receptacles from the motor caps of thesecond exemplary conventional BLDC motor, in accordance with theabove-described embodiments of the invention, reduces the diameter ofthe motor caps by approximately 3 mm. It was further found that usingthe inner diameter (ID) piloting and retention features, in accordancewith the above-described embodiments of the invention, further reducesthe diameter of the motor caps (not including the rectangular peripheraltabs 304) to approximately 51 mm—approximately the same diameter as theOD of the stator lamination stack. It is noted that the distance betweenthe rectangular peripheral tabs 304 in the first bearing support member250, as noted in Table A, is approximately 54.4 mm, which is only anapproximately 5% increase. However, the rectangular peripheral tabs 304are received in channels 322 of the housing and therefore do notcontribute to a considerable increase to the motor diameter.

Accordingly, in accordance with embodiments of the invention, given thesame space constraints in the power tool housing, a BLDC motor may beprovided with a larger stator lamination stack OD without increasing theoverall diameter of the motor. In the example above, the laminationstack OD was increased from 48 mm to 51 mm while maintaining the overalldiameter of the motor at no more than 54.4 mm. It was further found bythe inventors that such an increase to the OD of the stator laminationstack substantially increases power output, torque output, andefficiency. Specifically, it was found that given the same stator slotarea, stator slot fill, and stator lamination stack length, magnetgrade, lamination grade, and magnet length, and while maintaining themaximum no-load speed, increasing the stator lamination stack diameterin this matter results in an increase in the power output by 10% to 20%,particularly by approximately 15%; an increase in the torque output by40% to 55%, particularly by 30 to 40%, more particularly byapproximately 35%; and an increase in efficiency by approximately 5%.

Table B below sets forth the size and performance parameters of animproved BLDC according to embodiments of the invention. In anembodiment, given the space limitations set forth below and parametersprovided below, the motor of this invention outputs more than 900 W MaxOut power and over 0.80 Nm of torque at Max Watts out.

TABLE B 1st Conv. BLDC Improved BLDC Stator Lamination OD   48 mm   51mm First (Fan-Side) Motor Cap 54.4 mm 54.4 mm Stator Stack length   25mm   25 mm Rotor Stack Length 25.6 mm 25.6 mm Rotor Magnet Length   26mm   26 mm Slot Fill 36% 36% Magnet Grade 48H 48H Lamination Grade35H360 Nippon 35H360 Steel Nippon Steel No-Load Speed 20,500 rpm 20,500rpm Max Power (Max Watts 806 W 929 W Out) Max Efficiency 85% 89% Torqueat Max Watts Out 0.67 Nm 0.90 Nm

In the above-described first embodiment, the two bearing support members250 and 270 are provided as end caps arranged at the two ends of thestator assembly 230. A second alternative embodiment of the invention isdescribed herein with reference to FIGS. 12-15B.

FIG. 12 depicts a perspective view of a BLDC motor 400, according to anembodiment. FIG. 13 depicts a partially exploded view of the same motor400, according to an embodiment.

Similarly to the first embodiment, motor 400 is a three-phase motorincluding a rotor assembly 410 rotatably received within a statorassembly 430.

In an embodiment, stator assembly 430 is similar to and includes many ofthe same features as stator assembly 230 of the first embodiment. Toprovide an overview, stator assembly 430 includes a generallycylindrical lamination stack 434 having a center bore and a plurality ofteeth extending inwardly from the cylindrical portion of the laminationstack 434 defining a plurality of slots. Stator windings 434 are woundaround the stator teeth within the adjacent slots. Front and rear endinsulators 436 and 437 are provided on the end surfaces of the statorlamination stack 432 to insulate the lamination stack 432 from thestator windings 434. Front and rear end insulators 436 and 437 may beshaped to be received at the two ends of the stator lamination stack432. In an embodiment, each end insulator 437 and 436 includes a radialplane that mates with the end surfaces of the stator lamination stack432, and teeth and slots corresponding to the stator teeth and statorslots. Unlike the embodiment of FIGS. 3A and 3B, in the illustrativeembodiment, terminals 438 are disposed at an axial end of the statorassembly 230, with the rear end insulator 436 including retentionfeatures disposed circumferentially around the end of the statorassembly 430 for holding the terminals 438.

FIG. 14 depicts an exploded perspective view of a sub-assembly includingthe rotor assembly 410 and bearing support members 450 and 470. Rotorassembly 410 is similar to and includes many of the same features asrotor assembly 210 of the first embodiment. To provide an overview, asshown in FIG. 14 , and with continued reference to FIGS. 12 and 13 ,rotor assembly includes a rotor shaft 412, a rotor lamination stack 414having a series of flat laminations, and permanent magnets 415 disposedwithin axial slots of the lamination stack 414. In an embodiment, a fan418 is mounted on one distal end of the rotor shaft 412, and a pinion415 is disposed one the other distal end of the rotor shaft 412 forengagement with the transmission assembly 114. In an embodiment, tworotor caps 411 are disposed on the two ends of the rotor laminationstack 414 to axially restrain the magnets 415 within the laminationstack 414. In an embodiment, rotor end caps 411 may be similar to endcaps 226 shown in FIG. 3B and described later in this disclosure ingreat detail. In an embodiment, sense magnet 416 is disposed on therotor 412 via a bushing 417 between the second bearing support member470 (described below) and the pinion 415.

In an embodiment, the first bearing support member 450 is disposed on arear side of the rotor assembly 410 between the rotor assembly 410 andthe fan 418. In an embodiment, first bearing support member 450 includesmany of the same features as the first bearing support member 250previously described, including axial post inserts 280 projecting from aplanar radial body and slots 456 formed between respective teeth thatallow passage of airflow generated by the fan 418 through the statorassembly 430. A center portion of the first bearing support member 450defines a bearing pocket that receives the rear rotor bearing 420therein. In this example, the first bearing support member 450 is notprovided with a hall board. It must be understood, however, that thesense magnet 416 may be disposed between the rotor lamination stack 414and the first bearing support member 450 (e.g., in place of thecorresponding rotor end cap 411), and the first bearing support member450 may be provided with a hall board, as previously described withreference to FIGS. 3B and 4 .

The second bearing support member 470 is described herein with referenceto FIGS. 15A and 15B, and with continued reference to FIGS. 12-14 . Inan embodiment, second bearing support member 470 is shaped andconfigured to be received axially through the stator assembly 430. Thesecond bearing support member 470, in an embodiment, includes asubstantially disc-shaped planar portion 476 having a through-hole 474in its center portion therein forming a bearing pocket for receiving thefront rotor bearing 422 therein. The second bearing support member 470further includes a series of axial post inserts 472 disposed axiallyaround and attached to the circumference of the planar portion 476 via aseries of radial connection members 473. In an embodiment, the axialpost inserts 472 are sized such that a peripheral portion 471 of theaxial post inserts 472 engage an inner surface of stator laminationstack 432 slots to hold and radially restrain the second bearing supportmember 470 within the stator lamination stack 432.

Unlike the previous embodiment, the second bearing support member 470does not include a mating surface that mates with an outer surface ofthe stator assembly. This allows the second bearing support member 470to traverse through the length of the stator lamination stack 432, withthe axial post inserts 472 engaging and forcefully sliding against theinner surface of stator lamination stack 432 slots.

During the assembly process, in an embodiment, the rear and frontbearings 420 and 422 are received with the bearing pockets of the firstand second bearing support members 450 and 470. The first and secondbearing support members 450 and 470 are then mounted (e.g., viapress-fitting) onto the rotor shaft 412 on two sides of the rotorassembly 410, sandwiching the rotor assembly 410 on its two ends, toform the sub-assembly shown in FIG. 14 . The fan 418, sense magnet 516,and pinion 405 may also be mounted on the rotor shaft 412 as shown inFIG. 14 .

In an embodiment, the entire sub-assembly including the rotor assembly410 and the first and second bearing support members 450 and 470 maythen be axially received within the stator assembly 430, from the rearside of stator assembly where the rear end insulator 437 is located,until the rear end insulator 437 comes into contact with a matingsurface of the first bearing support member 450. As the assembly isbeing inserted into the stator assembly 430, the axial post inserts 472are forced against the inner surface of the stator lamination stack 432slots, within the gaps between adjacent stator windings.

In an embodiment, similarly to the embodiment described above, the rearend insulator 437 and the first bearing support member 450 may includecorresponding indentations and detents that mate together to help pilotand locate the two sub-assemblies. The rear end insulator 437 and thefirst bearing support member 450 also include peripheral tabs and otherengagement features (e.g., U-shaped walls 408) for piloting andplacement of the motor 400 within a power tool housing.

In an embodiment, the front end insulator (see FIG. 12 ) includes axialchannels or recesses 452 that receive the end portions of the axial postinserts 472 of the second support member 470 therein as the secondsupport member 470 is being axially pressed through the lamination stack438.

In an embodiment, the above-described arrangement provides a motorassembly 400 in which one of the rotor bearings (i.e., front bearing422) that radially supports the rotor assembly 410 within the statorassembly 430 is structurally supported fully within the stator assembly430, i.e., on the inner diameter of the stator lamination stack 432 andthe front end insulator 436. This arrangement significantly eases themanufacturing process, allowing both bearings 420, 422 to be mounted onthe rotor shaft 412 prior to assembly of the rotor 410 into the statorassembly 430.

In addition, with this arrangement the motor 400 is provided without afront motor end cap for supporting the front bearing 422, thus reducingthe length of the motor 400 by several millimeters. In an embodiment, asshown in FIG. 12 , the front bearing 422 (hidden behind the sense magnet416) may be positioned along approximately the same radial plane as thefront end insulator 436 and/or the ends of the stator windings 434.Thus, the supporting structure for the front bearing 422 does not add tothe overall length of the motor 400.

The third embodiment of the invention is described herein with referenceto FIGS. 16-19 . In this embodiment, both front and rear bearing supportstructures are fully supported within the stator assembly 430.

FIG. 16 depicts a perspective view of a BLDC motor 500 having internallysupported bearing support structures, according to an embodiment. FIG.17 depicts a partially exploded view of the same motor 500, according toan embodiment.

FIG. 18 depicts a perspective view of a sub-assembly including the rotorassembly 410, the second bearing support member 470 as described abovewith reference to FIGS. 12-15B, and an alternative and/or improved firstbearing support member 550, according to an embodiment. FIG. 19 depictsan exploded view of the sub-assembly shown in FIG. 18 .

In an embodiment, most components of the motor 500 of this embodimentare similar to motor 400 of the second embodiment described above, withthe exception of the first bearing support member 550, described here.

In an embodiment, first bearing support member 550, similarly to thesecond bearing support member 470, is shaped and configured to beaxially received and pressed through the stator assembly 430. The firstbearing support member 550, in an embodiment, includes a substantiallydisc-shaped planar portion 556 having a through-hole 554 in its centerportion therein forming a bearing pocket for receiving the rear rotorbearing 520 therein. The first bearing support member 550 furtherincludes a series of axial post inserts 552 disposed axially around andattached to the circumference of the planar portion 556 via a series ofradial connection members. In an embodiment, the axial post inserts 552are sized such that their peripheral portions engage an inner surface ofstator lamination stack 432 slots to hold and radially restrain thefirst bearing support member 550 within the stator lamination stack 432.

In an embodiment, the first bearing support member 550 does not includea mating surface that mates with an outer surface of the stator assembly430 (i.e., at end insulator 437). This allows the first bearing supportmember 550 to be received into the stator lamination stack 432, with theaxial post inserts 552 engaging and forcefully sliding against the innersurface of stator lamination stack 432 slots.

In an embodiment, axial post inserts 552 may have a generallyrectangular cross-sectional profile extending from a peripheral portion562 (see FIG. 18 ), which is arranged to engage an inner surface of acorresponding lamination stack slot, to an end portion 564, which may beslightly thicker that the peripheral portion 562 and is arranged to bedisposed at an open end of the lamination stack slot, between and inengagement with two adjacent stator tooth edges. In an embodiment, theperipheral portions 562 of the axial post inserts 552 align withperipheral portions of axial post inserts 472 of the second bearingsupport member 470.

In an embodiment, first bearing support member 550 may include a seriesof notches 566 (see FIG. 18 ) around bases of the axial post inserts552. Rear end insulator 437 may also include corresponding detents 570(see FIG. 17 ) that receive the notches 566 therein, and thus axiallyrestrain the first bearing support member 550, when rotor assembly 410and the two bearing support members 470 and 550 are fully inserted intothe stator assembly 430.

In an embodiment, the above-described arrangement provides a motorassembly 400 in which both the rotor bearings 422 and 520 that radiallysupports the rotor assembly 410 within the stator assembly 430 arestructurally supported fully within the stator assembly 430, i.e., onthe inner diameter of the stator lamination stack 432 and the front endinsulator 436, 437. With this arrangement the motor 400 is providedwithout front and rear motor end caps for supporting the front bearing422, thus reducing the length of the motor 400 significantly. In anembodiment, when fully assembled, the front bearing 422 may bepositioned along approximately the same radial plane as the front endinsulator 436 and/or the front ends of the stator windings 434.Similarly, the rear bearing 520 may be positioned along approximatelythe same radial plane as the rear end insulator 437 and/or the rear endsof the stator windings 434. Thus, the supporting structures for the rearand front bearings 420 and 422 do not add to the overall length of themotor 400.

Another aspect/embodiment of the invention is described herein withreference to FIGS. 20A and 20B.

As previously described, the rotor permanent magnets are axiallycontained within the rotor lamination stack via two rotor end cap onboth sides, or via a rotor end cap on one side and a sense magnet discon the other. The rotor end cap may be a disc-shaped plate, as shown inthe exemplary embodiments of FIGS. 14 and 19 (see end cap 411).Alternatively, as shown in FIGS. 3B and 6 , an improved rotor end cap226 may be provided to improve thermal transfer and cooling of the rotorlamination stack 210, according to an embodiment.

FIGS. 20A and 20B depict front and back perspective views of rotor endcap 226, according to an embodiment. As shown in these figures, rotorend cap 226 may include a peripheral planar portion 606 that mounted onaxial end(s) of the rotor lamination stack, a center bore 604 throughwhich the rotor shaft is received, and a series of ribs 608 disposed atan angle with respect to the plane of the planer portion 606 that extendfrom the planar portion 606 to a frontal end 605 of the center bore 604.A series of axial openings 602 are formed between the ribs 608, whichallow air to come into contact with the end of the rotor laminationstack, including the permanent magnets. Further, the angular dispositionof the ribs 608 allows the air to circulate tangentially betweenadjacent openings 602 under the ribs 608 and in contact with the end ofthe lamination stack.

In an embodiment, one or more inner walls 603 extend from the planarportion 606 to a rear end 607 of the central bore. The planar portion606, together with the inner walls 603, engage at least a portion of theend of each of the permanent magnets, ensuring that the permanentmagnets are fully axially retained within the rotor lamination stack.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

The invention claimed is:
 1. A brushless direct-current (DC) motorcomprising: a stator assembly including a stator core, a plurality ofteeth extending radially-inwardly from the stator core, and a pluralityof windings wound around the plurality of teeth; and a rotor assemblyrotatably received within the stator assembly, the rotor assemblyincluding a rotor shaft, a rotor body mounted on the rotor shaft andincluding a plurality of axial slots, a plurality of permanent magnetsdisposed within the axial slots, and a rotor end cap provided at an endof the rotor body to axially retain the plurality of permanent magnetswithin the plurality of slots, wherein the rotor end cap comprises anouter planar portion that is in contact with the end of the rotor body,a plurality of ribs angularly extending from the outer planar portiontowards a center portion forming a plurality of openings therebetween toallow passage of air between at least two of the adjacent openings inthermal contact with the end of the rotor body, and a plurality of innerwalls extending from the center portion to the outer planar portion incontact with at least a portion of the plurality of permanent magnets toaxially retain the plurality of permanent magnets within the rotor body.2. The motor of claim 1, wherein the plurality of inner walls is formedalong a first subset of the plurality of ribs while a second subset ofthe plurality of ribs is at least partially axially spaced apart fromthe end of the rotor body to allow passage of air between the adjacentopenings.
 3. The motor of claim 1, wherein the plurality of openings ofthe rotor end cap comprises six openings and the plurality of innerwalls of the rotor end cap comprises three legs to allow flow of airbetween pairs of openings.
 4. The motor of claim 1, wherein the planarportion is in contact with at least a portion of the plurality ofpermeant magnets to axially retain the plurality of permanent magnetswithin the rotor body.
 5. The motor of claim 1, further comprising asense magnet dispose on a second end of the rotor body opposite therotor end cap.
 6. The motor of claim 5, wherein a rear surface of thesense magnet engages the plurality of permeant magnets to axially retainthe plurality of permanent magnets within the rotor body.
 7. The motorof claim 1, further comprising a fan mounted on the rotor shaft togenerate an airflow through the motor.
 8. A power tool comprising: ahousing; a drive shaft; and a brushless direct-current (DC) motordisposed within the housing for driving the drive shaft, the motorcomprising: a stator assembly including a stator core, a plurality ofteeth extending radially-inwardly from the stator core, and a pluralityof windings wound around the plurality of teeth; and a rotor assemblyrotatably received within the stator assembly, the rotor assemblyincluding a rotor shaft, a rotor body mounted on the rotor shaft andincluding a plurality of axial slots, a plurality of permanent magnetsdisposed within the axial slots, and a rotor end cap provided at an endof the rotor body to axially retain the plurality of permanent magnetswithin the plurality of slots, wherein the rotor end cap comprises anouter planar portion that is in contact with the end of the rotor body,a plurality of ribs angularly extending from the outer planar portiontowards a center portion forming a plurality of openings therebetween toallow passage of air between at least two of the adjacent openings inthermal contact with the end of the rotor body, and a plurality of innerwalls extending from the center portion to the outer planar portion incontact with at least a portion of the plurality of permanent magnets toaxially retain the plurality of permanent magnets within the rotor body.9. The power tool of claim 8, wherein the plurality of inner walls isformed along a first subset of the plurality of ribs while a secondsubset of the plurality of ribs is at least partially axially spacedapart from the end of the rotor body to allow passage of air between theadjacent openings.
 10. The power tool of claim 8, wherein the pluralityof openings of the rotor end cap comprises six openings and theplurality of inner walls of the rotor end cap comprises three legs toallow flow of air between pairs of openings.
 11. The power tool of claim8, wherein the planar portion is in contact with at least a portion ofthe plurality of permeant magnets to axially retain the plurality ofpermanent magnets within the rotor body.
 12. The power tool of claim 8,further comprising a sense magnet dispose on a second end of the rotorbody opposite the rotor end cap.
 13. The power tool of claim 12, whereina rear surface of the sense magnet engages the plurality of permeantmagnets to axially retain the plurality of permanent magnets within therotor body.
 14. The power tool of claim 8, further comprising a fanmounted on the rotor shaft to generate an airflow through the motor. 15.A brushless direct-current (DC) motor comprising: a stator assemblyincluding a stator core, a plurality of teeth extendingradially-inwardly from the stator core, and a plurality of windingswound around the plurality of teeth; and a rotor assembly rotatablyreceived within the stator assembly, the rotor assembly including arotor shaft, a rotor body mounted on the rotor shaft and including aplurality of axial slots, a plurality of permanent magnets disposedwithin the axial slots, a rotor end cap provided at an end of the rotorbody to axially retain the plurality of permanent magnets within theplurality of slots, and a sense magnet dispose on a second end of therotor body opposite the rotor end cap; wherein the rotor end capcomprises an outer planar portion that is in contact with the end of therotor body, and a plurality of ribs angularly extending from the outerplanar portion towards a center portion forming a plurality of openingstherebetween to allow passage of air between at least two of theadjacent openings in thermal contact with the end of the rotor body, andwherein a rear surface of the sense magnet engages the plurality ofpermeant magnets to axially retain the plurality of permanent magnetswithin the rotor body.